it's to help you check for the smallest clearance used if you have different schemes.
Select master rule area -> right click -> select clone or new to make a new scheme
In expedition PCB add a rule area to use the new scheme in a zone like f.e. a BGA fanout.
The Master Scheme represents the Default constraints for Clearances and Trace and Via Properties for the entire design. These values will be used if no unique constraints or rule areas are applied.
The Minimum Scheme is read-only. It represents an overview of the minimum values entered for Clearances and Trace and Via Properties across all of the schemes within the design. It provides the user with an easy method to review any entered constraints that may be unacceptable.
I hope this clears it up for you.
Thank you for your replies. Now I got it